In all but disabled drivers, the right and left hands provide the interface between the central nervous system and the machine's guidance or steering apparatus. Over the last fifty years, technical modifications of automotive steering systems have focused upon the linkage system, airbag deployment and decorative wheel designs. In only a few cases have design changes been directed toward hand positioning and comfort as it relates to ergonomics and safety.
A non-consecutive survey of 1000 drivers was completed, recording hand positions from drivers on highways in Tampa, Fla. Two surveyors observed the hand position on the steering wheel for drivers at speeds greater than 30 miles per hour. While alongside the subject's car, the position of the hand(s) on the steering wheel was observed. A predetermined code was dictated into a recorder based on the household clock orientation. Seventy five percent (750) of the observations returned only one hand on the steering wheel. Right and left hands are depicted in the chart of FIG. 2.
In the study, when two hands guided the automobile, they were balanced in 90% of the observations. When one and two hand positions were combined, 89% were noted at or above the horizontal midline (9 o'clock to 3 o'clock). The most common right one hand position was at 11 or 12 o'clock (75%). Thirty two percent of the left hands (one hand grip) were placed at 12 o'clock. The single left hand was positioned at 12, 11, 10 or 9 o'clock in 76% of the observations. Forty-one percent of the one hand steering grips crossed the vertical midline. Anytime the hand crosses the vertical midline to grasp the steering wheel, a potential dangerous situation occurs related to airbag deployment.
The forearm rotates from supination (palm up) (shown in FIG. 4) to pronation (palm down) (shown in FIG. 5). The mid-forearm position of rotation is termed neutral (shown in FIG. 3). For purposes of the descriptions herein, the forearm positions are described with the upper arms hanging loosely at the sides of the upper trunk. It is this reference that closely compares to the normal driving position, i.e., arms hanging loosely at the sides of the upper trunk.
At the wrist joint, the hand extends upward, flexes downward and deviates side-to-side. Ulna deviation occurs when the hand moves toward the small finger side in the plane of the forearm. Radial deviation is the opposite and the hand moves toward the thumb side in the plane of the forearm. Wrist movement is complex and brought about by muscle induced shifts and rotations of the small bones of the carpus which span between the forearm and hand. Carpal shifts and rotations are almost the same in wrist extension (shown in FIG. 7) and ulnar deviation (shown in FIG. 8). Likewise, the carpal movements are similar in wrist flexion (shown in FIG. 6) and radial deviation (shown in FIG. 9).
Industry has expended significant resources on ergonomic issues over the past 15 years. Most new technologies related to the upper extremity were directed toward design modifications of tools and input devices. Hardware stores display the advantages of multi-geared wrenches, angled saws, and hand-friendly drivers. High-tech retailers are very active in the promotion of ergonomic input devices for the home and office computer.
Most ergonomic and safety devices have been developed to diminish stresses in the extremity created by repetitive activity and static holding. Experimental research has shown well the harmful effects of wrist and forearm positions during holding and static hand positioning. Ergonomic experts recommend avoiding hand and wrist positions that create tension across joints and increase pressure on the tightly compartmentalized median nerve at the base of the palm. Pronation of the forearm, ulnar deviation of the palm and flexion of the wrist increase discomfort in users. Tissue tensions are high, which creates increased contact pressures in the joints and intraneural pressures across the median nerve within the carpal canal. Discomfort is usually experienced as joint aching and hand numbness. These symptoms are influenced by many exogenous factors, such as age, obesity, diabetes, arthritis and preexisting neck, shoulder and extremity pathology.
Technical modifications of computer interface devices that change the static hand(s) position(s) have filled retail stores. On many input boards the keys are angles outward to reduce ulnar deviation in the wrist. Portions of the boards are rolled or humped to reduce the amount of pronation of the forearms. The “vertical keyboard” takes this design to the extreme and is comprised of two half boards that permit entry with the forearms in the neutral position.
The most common situation that potentially fixes the hand(s) in a static position for varied time intervals occurs during driving. The forearm is fully pronated and the wrist is significantly ulnar deviated in the ten-two o'clock hands position. When the driver's hand crosses the vertical midline (12 o'clock to 6 o'clock), the forearms remain pronated but ulnar deviation is reversed to neutral. The limb, however, is in an unsafe position as it will be impacted by airbag deployment. Orthopedic, plastic, and hand surgeons are familiar with injuries to the hand and face that occur in front end collisions.
FIG. 10 shows the standard ten two hand position on the steering wheel. Forearms are fully pronated and the wrists are ulnar deviated. FIGS. 11 and 12 demonstrate the left hand on either side of the vertical equator of the steering wheel. Although the left two o'clock position straightens the ulnar deviated wrist, the position is potentially dangerous during airbag deployment.
During collision, the airbag drives the left hand that was held across the center of the wheel into the face of the driver, as shown in FIG. 13. It would be advantageous to have available a steering wheel design that allows the hands to be set in a more relaxed position for better comfort, and at the same time produce a safer driving environment. This steering wheel design should promote a more neutral position of the forearms and discourage fixed wrist positions in flexion and ulnar deviation. Additions to the commonly used steering wheels have been attempted but may add numerous other difficulties, including problems with actual steering and other safety issues.